It is often necessary to illuminate objects, scenes, tissue or biological samples, chemical compositions and other items with light for the purpose of capturing an image, making a measurement, or initiating a chemical or biological reaction. For all of these purposes it is important to control the amount of light illuminating the object or scene. The most common ways to control the amount of illumination are by varying the output energy of the source of illumination or controlling the duration of illumination.
For many sources of illumination it is not practical or possible to vary the output energy. For other sources the output energy can be varied but without adequate precision.
Shutters are the most common method of controlling the amount of illumination by controlling the duration of illumination. Shutters are well known and control the duration of illumination of imaging sensors as well as the duration of illumination of an object or scene being imaged.
A shutter is a device that has two states—open and closed. In the open state it allows light to propagate along an optical path. In the closed state it blocks the optical path and prevents light from propagating. In other words, shutters open to let light through and close to keep the light out. Typical shutters may be mechanical, electromechanical or solid-state. Electromechanical shutters are often operated under microprocessor control to determine the duration of their opening and closing.
In many applications illumination needs to be turned on and off repeatedly at high speed. For these applications a shutter with multiple apertures arranged circularly in a spinning disk is often used. Such a shutter is commonly known as a “chopper wheel”. A problem with chopper wheels is that it is impossible to vary the duration of exposure without changing the frequency of exposure and vice versa.
A problem with digital light processors (DLPs, also known as pixelated spatial light modulators (SLMs)) has been their lack of contrast. While most of the energy impinging on the array of mirrors is controllably reflected by the mirrors of the DLP, a small amount of the light is imperfectly reflected by the small deformations in the mirrors and by impinging on the electrical and mechanical components between and below the mirrors. This results in a small amount of undesired light scattering from these surfaces and along the propagating path. The contrast ratio of these devices has historically been about 400:1. Recent improvements have raised the contrast ratio to about 1000:1. The quality of the contrast ratio is very dependent on the angle at which light impinges on the DLP and can be reduced when light strikes at multiple angles.
Thus, a limitation of using single DLPs as shutters is that they still pass a small amount of light even when turned “off” making them unsuitable for a number of applications.
In many applications currently available electromechanical shutters cannot provide sufficient speed of actuation or precision of duration. One reason for this is that many electromechanical shutters comprise metal leaves that are moved by the action of small solenoids, triggered by electrical signals. Although these components are small and light they have a certain amount of inertia that must be overcome, before they can be moved to block or unblock the passage of light. While this time is short, there is a finite amount of time that must pass while the shutter transitions between opened and closed states. This limits the precision of many of these types of shutter when exposure times approach 100 milliseconds ( 1/10 of a second) or less.
This also limits the shortest exposure time that can be practically achieved.
One attempt to provide this level of exposure control has been to use a solid-state device such as a liquid crystal as a shutter. Liquid crystals can change their polarization in response to an electrical signal. When polarized light is directed onto the crystal it passes through the liquid crystal when the liquid crystal has the same polarization, but is blocked when the polarization is in an orientation that opposes the passage of light.
Liquid crystal shutters require that the illumination light impinging on them be polarized to be controllable which reduces the output power of many sources, and furthermore makes them unsuitable for applications where polarized light is not desirable. Liquid crystal shutters tend to overheat because they must also absorb the light that they do not pass, and therefore have the disadvantage of having to manage the dissipation of the absorbed light, making them unsuitable for many applications where higher power light sources are required.
There has gone unmet a need for a shutter to precisely control the exposure time of illumination, that operates at high speed, that is not limited to polarized light and that can accommodate higher power light sources. The present invention provides these and other advantages.